1. Field of the Invention
The present invention relates to a projection type color liquid crystal (LC) display apparatus for projecting an image onto a screen.
2. Description of the Related Art
Image display apparatuses are available which display an image on a cathode ray tube (CRT) display and project the image on the CRT display onto a screen. In recent years, however, projection type color LC display apparatuses (which incorporate LC display devices) have been proposed to replace such projection type display apparatuses. There is a class of projection type color LC display apparatuses called a "single plate type" (e.g., as disclosed in Japanese Laid-open Publication No.4-60538) which incorporate a single LC display device. Single plate type projection type color LC display apparatuses are known as especially suitable for use in systems with small dimensions due to their relatively simple optical systems.
A single plate-type projection type color LC display apparatus is illustrated in FIG. 11. As shown in FIG. 11, light from a incandescent light source 101 is reflected from a mirror 102 (shaped as an ellipsoid of revolution), which is disposed in such a manner that its first focal point coincides with the location of the light source 101, and converges onto the second focal point of the mirror 102.
The light converged at the second focal point of the mirror 102 is passed through an integrator 103 (which is located near the second focal point of the mirror 102), whereby it becomes uniform in terms of its angular distribution, i.e., dispersed. This light is further collimated by means of a condenser lens 104. The collimated light is separated by dichroic mirrors 105R, 105G, and 105B (having selective reflectivity for red, green, and blue, respectively) into respective components of three primary colors, i.e., red (R), green (G), and blue (B). These light beams of three colors enter respective picture elements of a LC display device 106 corresponding to R, G, and B.
The LC display device 106 includes: a LC panel 107 for modulating its incident light in accordance with an image signal; a microlens array 108 for converging the light beams of three colors onto the respective picture elements of the LC display panel 107 corresponding to R, G, and B; a polarizing plate 109 disposed on the side of the microlens array 108 from which the light enters (defined as the "incident side"); and a polarizing plate 110 disposed on the side of the LC panel 107 from which the light goes out (defined as the "outgoing side").
FIG. 12 illustrates the configuration of the LC panel 107, which includes a LC layer 113 interposed between a pair of opposing substrates 111 and 112. A plurality of pixel electrodes 115 are disposed in a matrix shape on the surface of the substrate 111. A thin film transistor (TFT) 116 and a storage capacitance 117 are provided corresponding to each pixel electrode 115 for driving the pixel electrode 115. One pixel is defined by a corresponding set of a pixel electrode 115, a TFT 116, and a storage capacitance 117.
Signal lines 118 are provided on the surface of the substrate 111 corresponding to respective columns of pixel electrodes 115, whereas gate lines 119 are provided corresponding to respective rows of pixel electrodes 115. A gate of each TFT 116 is coupled to a corresponding gate line 119; a source of the TFT 116 is coupled to a corresponding signal line 118; and a drain of the TFT 116 is coupled to a pixel electrode 115 and one of the electrodes of the storage capacitance 117. The other electrode of the storage capacitance 117 is set at a potential which is at the same level as that of a counter electrode 121. The substrate 111 thus constructed is commonly referred to as an "active matrix substrate".
On the counter substrate 112 opposing the active matrix substrate 111, a counter electrode 121 and a light shielding layer 122 are layered in this order. In positions of the light shielding layer 122 corresponding to the respective pixel electrodes 115, aperture regions 123 exist. The LC molecules in the LC layer 113 may take, for example, a twisted nematic orientation state.
FIG. 13 illustrates a cross section of the LC panel 107 and the microlens array 108. As seen from FIG. 13, the microlens array 108 includes microlenses 124 of a hexagonal shape wherein the outer peripheries of individual spherical lenses are merged with one another. Such a microlens array 108 can be produced by an ion exchange method.
The microlenses 124 are located relative to the picture elements 125 as shown in FIG. 14. Specifically, on the LC panel 107, repetitive sets of picture elements 125 of R, G, and B (arranged in this order) are provided in each horizontal line of the display. Furthermore, with respect to any two adjoining horizontal lines, each picture element 125 in the upper (lower) horizontal row is offset from the corresponding element 125 in the lower (upper) horizontal row by substantially half of the pitch of the picture elements 125. The microlenses 124 are disposed so that the optical axis of each microlens 124 coincides with the center of the "green" picture element 125G of the corresponding set of picture elements 125R, 125G, and 125B.
The light beams of R, G, and B entering the microlenses 124 thus disposed are converged to form convergence spots that fall within the aperture regions 123 of the picture elements 125 of the corresponding colors.
Referring back to FIG. 14, among the components of the light beams R, G, and B which have been reflected from the dichroic mirrors 105R, 105G, and 105B at their respective angles, only the P-polarization component passes through the polarizing plate 109, while the S-polarization component is absorbed by the polarizing plate 109.
The P-polarization component of the green light beam enters a microlens 124 along the direction of the normal axis of the microlens 124 (hereinafter referred to as the "normal direction"), so as to be converged on the aperture region 123 of the corresponding green picture element 125G. The P-polarization components of the red and blue light beams enter the microlens 124 at an angle .theta.(in opposite directions) with respect to the normal direction, so as to be respectively converged on the aperture regions 123 of the red and blue picture elements 125R and 125B flanking the green picture element 125G.
Thus, in the illustrated projection type color LC display apparatus, light beams of R, G, and B are respectively converged onto the corresponding picture elements 125R, 125G, and 125B of the LC display device 106. The light beams which have passed through the respective picture elements 125 are further projected onto a screen 128 via a field lens 126 and a projection lens 127, whereby color display is effected.
If the collimated light from the condenser lens 104 has a poor degree of parallelism in the above-described conventional color LC display apparatus, a portion of the collimated light may not be properly converged through the microlens array 108 to stay within the aperture region 123 (FIG. 12), and hence intercepted by the light shielding layer 122, i.e., not transmitted through the LC display device 106. As a result, the display images becomes darker.
The above-mentioned problem might appear to be overcome by simply optimizing the parallelism of the collimated light. However, such an approach has the following problems.
Since the parallelism of a collimated light beam generally decreases as the diameter of the collimated light beam decreases, the diameter of the collimated light beam must be maximized. However, there is an upper limit to the diameter of the collimated light beam. The reason is that an excessively large diameter of the collimated light beam causes peripheral portions of the collimated light beam to fall outside the dichroic mirrors 105R, 105G, or 105B and even the LC display device 106.
On the aspect of enhancement of the brightness of the displayed images, one may wish to increase the intensity of the light source 101. However, the larger the arc of the light source 101 becomes, the more difficult it becomes to maintain the parallelism of the collimated light beam.
Thus, under the prior art, it has been difficult to maintain the parallelism of the collimated light beam while employing a sufficiently bright light source 101, and hence to provide a bright displayed image.